I found a YouTube video demo of this app/app/driver combo that shows its use with FinalCut Pro X among other things.
A few points:

1. The claim of zero delay relies on a recent iPad processor or "regular" instead of "retina" display on the iPad.
Using an iPad 3, the demo showed some obvious delay when the app was set to retina. There was no delay, even at retina, when he switched to an iPad Air 2.
2. Apps like FinalCut, which have controls for what's shown on a second monitor, make this look really useful. For example, you can have the video playback window on the iPad, full screen, and all the rest of FinalCut's many, many panels and windows on the main monitor.
3. You can click and move things on the iPad with your mouse, as usual for a second monitor. But you can also click and move things on the iPad with your finger, using its touch capabilities. I still haven't seen anything showing drawing in Photoshop, for example.

The key to all this, as compared to older similar apps seems to be a combination of the driver (written by an ex-Apple engineer who worked on display code for both Mac and iPad) and the fact that this solution is wired, using a lightning or 30-pin USB cable, not WiFi.

Also, the iTunes Store page shows an image that implies that you can use this as the only monitor. In the photo, sitting on top of a Mac mini. That might be excellent for a mini as audio server, for example.

"Design is not just what it looks like and feels like. Design is how it works."
Steve Jobs